Isocyanates are produced in large volumes and serve mainly as starting materials for production of polyurethanes. They are usually prepared by reacting the corresponding amines with phosgene, using phosgene in a stoichiometric excess. The reaction of the amines with the phosgene can be effected either in the gas phase or in the liquid phase, wherein the reaction can be conducted batchwise or continuously (W. Siefken, Liebigs Ann. 562, 75-106 (1949)). There have already been multiple descriptions of processes for preparing organic isocyanates from primary amines and phosgene; see, for example, Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th ed. (1977), volume 13, p. 351 to 353, and G. Wegener et al. Applied Catalysis A: General 221 (2001), p. 303-335, Elsevier Science B. V. There is global use both of aromatic isocyanates, for example methylene diphenyl diisocyanate (MMDI—“monomeric MDI”), polymethylene polyphenylene polyisocyanate (a mixture of MMDI and higher homologs, PMDI, “polymeric MDI”) or tolylene diisocyanate (TDI), and of aliphatic isocyanates, for example hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI).
A distinction is generally drawn between two ways of conducting the process, namely reaction in the gas phase and reaction in the liquid phase.
It is a feature of the process regime in the gas phase, typically referred to as gas phase phosgenation, that the reaction conditions are chosen such that usually the amine, isocyanate and phosgene reaction components, but preferably all the reactants, products and reaction intermediates, are gaseous under the conditions chosen.
It is a feature of the process regime in the liquid phase, typically referred to as liquid phase phosgenation, that the reaction conditions are chosen such that at least the amine, crude isocyanate and phosgene reaction components, but preferably all the reactants, products and reaction intermediates, are in liquid form under the conditions chosen (which in this connection also includes the state of a gas physically dissolved in the liquid phase) in a suitable solvent and the solvent is separated from the crude isocyanate and recycled into the reaction circuit in purified or else unpurified form.
For the mixing of the amine with the phosgene in liquid phase phosgenation, it is possible to use a dynamic or static mixer; in gas phase phosgenation, the use of a mixing nozzle is advisable. An important factor is rapid and good mixing of the feedstocks, since the isocyanate formed, in the case of poor mixing of the feedstocks, reacts with amine which is then present in a local excess to give urea or other troublesome by-products of high viscosity or in solid form. This gives rise to caking of equipment and blockages (for example in the pipelines) which lead to unwanted cleaning outages and hence to poorer service lives of the plant, i.e. to suboptimal economic viability. Consequently, the focus in the isocyanate-preparing industry in the recent past was on the optimization of the mixing of the feedstocks, as can also be inferred from the multitude of publications. The following patent applications are cited here by way of example: CN 102527312 A, WO 2013/048873 A1, WO 2013/060836 A1, EP 2 077 150 A1, EP-A-1 555 258, WO 2006/108740 A1, WO 2010/015667 A1, or EP-A-1 526 129.
Modern industrial scale preparation of polyisocyanates is continuous, and the reaction is conducted as an adiabatic phosgenation as described, for example, in EP 1 616 857 A1. Unwanted deposits and by-products in the reactor are avoided through correct choice of reaction temperature and pressure. In the mixing space, a molar excess of phosgene relative to the primary amino groups should be ensured. A three-stage phosgenation line is described in EP 1 873 142 A1, in which the pressure between the first stage of a mixer and the second stage of a first phosgenation reactor remains the same or rises and, in the third stage, an apparatus for phosgene removal, the pressure is lower than in the second stage.
WO 2013/029918 A1 (WO '918 from now on) describes a process for preparing isocyanates by reacting the corresponding isocyanates with phosgene, which can be conducted at different loads on the plant without any problems. More particularly, in the case of operation of the plant within the partial load range as well, the mixing and/or the reaction is to be effected within the dwell time window optimized in each case, by increasing the ratio of phosgene to amine or adding one or more inert substances to the phosgene and/or amine stream. The process of the invention is to enable operation of an existing plant at different loads with constant product and process quality. The intention is to dispense with the provision of several plants with different nameplate capacities. The following findings are made: both in the gas phase phosgenation and in the liquid phase phosgenation, the mixing of the reactants and the dwell time of the reaction mixture in the corresponding reaction spaces are critical reaction parameters. The plans for preparation of isocyanates by phosgenation of amines therefore have to be matched to the specific demands with respect to rapid mixing of the reactant streams and narrow dwell time windows. Plans for phosgenation of amines are designed here essentially for the maximum flow rates or the respective nameplate capacity. This means that both mixing elements, such as nozzles, and the reaction spaces, for example dwell time reactors, at the nameplate capacity, work within the optimal range with optimized yield, purity of the products, etc. However, if the plant is not at full load, meaning that it is being operated only at a fraction of the nameplate capacity, there will be a change, for example, in the dwell times, and the plant as a result is no longer working within the optimal range. This is the case, for example, in startup and shutdown operations, at partial load of the plant and in the event of faults in the plant. In these cases of reduced production capacity, both the mixing elements and the dwell time reactors are not working within the optimal range. The consequences are yield losses, problems with caking in equipment and/or losses of quality. In order to avoid the aforementioned problems, WO '918 suggests that sufficiently rapid mixing of the reactants should be ensured. Methods of implementing short mixing times are known in principle. In the mixing units, it is possible to use mixing aggregates having dynamic or static mixers. Preference is given to using one or more static mixing elements in the mixing units. Suitable static mixing elements include, for example, nozzles, smooth jet nozzles or Venturi nozzles that are known from combustion technology, and Laval nozzles. WO '918 suggests, as a particularly advantageous execution of a static mixing element, a mixing element as described in WO 2010/015667 A1. Dynamic mixers used may, for example, be rotor/stator systems disposed in the mixing units. According to WO '918, preference is given to using static mixing elements, especially nozzles. However, the process according to WO '918 solves these problems (yield losses, fouling problems and/or losses of quality) at the cost of use of elevated amounts of solvent and/or of elevated amounts of phosgene and not through the use of suitable nozzles. This means that, under partial load of the plant, the dwell time in the reactors and apparatuses in the plant is kept the same or nearly the same compared to the operation of the plant as intended at nameplate capacity. This gives rise to serious drawbacks, for example a higher specific and possibly absolute phosgene holdup or use of an elevated amount of solvent under partial production load. This is of course associated with the higher energy expenditure for the workup of the crude isocyanate, where more solvent than necessary has to be distilled and more phosgene than necessary recovered, i.e. condensed. As a result, the economic viability of the process suffers in addition to the load already being low.
The problems of optimal mode of operation at different load states that are discussed in WO '918 gives reason for some fundamental considerations:
The operating of a production plant both in the region of nameplate capacity (also referred to as “nameplate load”) and in the region of reduced production capacity (also referred to as “partial load range”) should each be considered to be a steady state in that the flow rates of reactants (amine, phosgene, optionally diluent), once set, remain constant. In order to change from a steady state (for example production at nameplate capacity) to a different steady state (for example production at 75% of nameplate capacity), however, a transition state is inevitably passed through each time, in which the flow rates of reactants are subject to constant changes, at the end of which the new steady state is established. In such transition states, the flow rates are therefore changing, either continuously or within discrete intervals. WO '918 does not discuss the configuration of such transition states in detail. Instead, WO '918 is concerned merely with the optimal operating of the process in the steady states before and after a transition state.
Published specification DE 10 2009 032413 A1 is concerned with a process for preparing isocyanates in the gas phase, in which the phosgene recovery yield is increased by means of a particular process regime in the workup of the gas stream comprising hydrogen chloride and phosgene which forms in the phosgenation. This document is not concerned with transition states between two operating states with different load either.
Patent application WO 03/045900 A1 is based on the objective of providing a process for preparing isocyanates by phosgenation in the gas phase, by means of which both a high heat exchange area and, even though it is impossible to completely avoid formation of solids, a very long operating life of the production plant, especially a production plant on the industrial scale, is to be achieved. For this purpose, it is suggested that the reaction be conducted in a non-cylindrical reaction channel. This document is not concerned with transition states between two operating states with different load either.
Published specification DE 32 12 510 A1 describes a two-stage liquid phase process for preparing an organic isocyanate. This document is not concerned with transition states between two operating states with different load either.